1. Field of the Invention
The present invention relates to a method of forming contacts for a semiconductor device, and more particularly, to a contact hole having an improved profile that allows better metal step coverage.
2. Description of the Related Art
FIG. 1a to FIG. 1c are flow diagrams showing a prior art method of forming contacts for a semiconductor device.
Referring to FIG. 1a, an interlayer insulating film 14 is formed on a semiconductor substrate 10. The interlayer insulating film 14 can be formed on a lower metal line (not shown) which is formed on the semiconductor substrate 10. A photoresist layer is formed on the interlayer insulating film 14 and etched to form a predetermined configuration (i.e., photoresist layer pattern 18) to define a contact region.
Portions of the exposed interlayer insulating film 14 are etched using the photoresist layer pattern 18 as a mask. Etching the interlayer insulating film 14 is performed by wet-etching. Since the wet-etching process is usually isotropic, meaning that etching proceeds outward in all directions, the upper recess portion includes undercut portions 19 of the interlayer insulating film 14 beneath the photoresist layer pattern 18. As a result, the upper layer of the portions of the interlayer insulating film 14 exposed by the photoresist layer pattern 18 has a sloped profile.
Referring to FIG. 1b, subsequent to the above isotropic wet-etching process, an anisotropic dry-etching process is conducted on the interlayer insulating film 14 through the opening of the photoresist layer pattern 18 so as to further remove unwanted parts of the interlayer insulating film 14, thereby entirely exposing the upper surface of the semiconductor substrate 10 or the lower metal line (not shown). Since the dry-etching process is highly anisotropic, meaning that the etching proceeds in only one direction (here, the downward direction), the sidewall of the bottom recess formed by dry-etching has a substantially vertical profile, as shown in FIG. 1b.
The photoresist layer pattern 18 is then removed. The upper recess portion and the bottom recess portion in combination constitute the desired metal contact hole 20, as shown in FIG. 1c.
The upper sidewalls of the contact hole 20 have a sloped profile. This is to improve the step coverage of the deposited metal.
FIG. 2 is a cross-sectional view showing a contact hole which is formed by a conventional method described above.
Referring to FIG. 2, the contact hole 20 which is formed by sequentially performing the wet-etching and dry-etching (hereinafter "wet-dry-etching") has a relatively close distance 16a between the wet-etched-surface 15a of the interlayer insulating film 14 and gate electrode layers 12, thereby reducing margins of interlayer insulating film 14 for isolating devices. That leads to a limited degree of integration.
To enlarge the margins for isolating devices, a method which employs sequentially performing dry-etching, wet-etching, and dry-etching (hereinafter "dry-wet-dry-etching") has been widely used in the art.
A contact hole which is formed by the dry-wet-dry-etching method has an etched-surface as illustrated in FIG. 2 as dotted lines, identified with reference number 15b.
The distance 16b between the gate electrode layers 12 and the etched-surface produced by the dry-wet-dry-etching method is relatively great compared with the distance 16a produced by the wet-dry-etching method.
However, considering the recent trend toward high density of integration, the drywet-dry-etching method also presents a limit.